The shafts equipped with the feed screw segments and the working segments can also be arranged in a hollow space in the extruder housing along a circle with the same central angle (EP 0 788 867 B1). Whilst the feed screw segments formed from individual elements feed the material to be processed in the extruder from the material feed opening to the material exit opening at the other end of the extruder, at the same time the working segments shaped as individual elements have a braking and, if necessary, a reversing function. As the working elements, kneading blocks, as known from EP 0 422 272 A1, for example, can be used, or so-called blisters, i.e. baffle plates with a diameter of the same size as the outer diameter of the screw, which can also be equipped as a so-called toothed disk with external teeth.
Instead of such working elements that compress the material, pressure relieving working elements are also used. For example, from DE 102 33 213 A1, a screw element is known that has a feed-neutral, pressure-relieving section which is formed through wearing of the screw at the screw cog.
The working elements, which have different surfaces for different process tasks, can be combined with each other in almost any way to optimally take into account the respective total technical process requirements. The feed screw elements and working elements are lined up tightly to each other and are placed on the carrier shafts in a torque-proof manner and, positioned accurately both radially and axially, are driven in a co-ordinated manner by the drive.
Due to special technical process requirements, working elements are frequently reduced in length to up to one sixth of the screw diameter and are almost always shorter than the screw diameter. On the other hand, high pressure can occur between the interlocking working elements of two adjacent shafts, particularly if these elements, such as reversing screw elements, kneading blocks, blisters or toothed disks, have a diameter that corresponds to the external sectional diameter. In this way, forces occur in double screw extruders around such working elements that lead to a considerable spread between the shafts. These forces also lead to considerable wear in multiple shaft extruders where the shafts are arranged around a circle with the same central angle.